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# Chiral control of quantum states in non-Hermitian spin–orbit-coupled fermions

## Abstract

Spin–orbit coupling is an essential mechanism underlying quantum phenomena such as the spin Hall effect and topological insulators1. It has been widely studied in well-isolated Hermitian systems, but much less is known about the role dissipation plays in spin–orbit-coupled systems2. Here we implement dissipative spin–orbit-coupled bands filled with ultracold fermions, and observe parity-time symmetry breaking as a result of the competition between the spin–orbit coupling and dissipation. Tunable dissipation, introduced by state-selective atom loss, enables us to tune the energy gap and close it at the critical dissipation value, the so-called exceptional point3. In the vicinity of the critical point, the state evolution exhibits a chiral response, which enables us to tune the spin–orbit coupling and dissipation dynamically, revealing topologically robust chiral spin transfer when the quantum state encircles the exceptional point. This demonstrates that we can explore non-Hermitian topological states with spin–orbit coupling.

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## Data availability

The data that support the findings of this work are available from the corresponding authors upon reasonable request.

## Change history

• ### 28 January 2022

In the version of the Supplementary Information originally published, there was a typo in the caption to Figure S1. The error has been amended and the Supplementary Information now replaced.

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## Acknowledgements

G.-B.J. acknowledges support from the RGC and the Croucher Foundation through grants nos. 16305317, 16304918, 16306119, 16302420, C 6005-17G and N-HKUST601/17. G.-B.J is further supported by the Harilela foundation. J.L. acknowledges support from the RGC through grants nos. 16304520 and C6013-18G.

## Author information

Authors

### Contributions

Z.R., E.Z., C.H. and K.K.P. carried out the experiment and data analysis and helped with numerical calculations. D.L. performed theoretical calculations. G.-B.J. and J.L. and supervised the research.

### Corresponding authors

Correspondence to Jensen Li or Gyu-Boong Jo.

## Ethics declarations

### Competing interests

The authors declare no competing interests

## Peer review

### Peer review information

Nature Physics thanks Wei Yi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–6 and Discussion.

## Rights and permissions

Reprints and Permissions

Ren, Z., Liu, D., Zhao, E. et al. Chiral control of quantum states in non-Hermitian spin–orbit-coupled fermions. Nat. Phys. 18, 385–389 (2022). https://doi.org/10.1038/s41567-021-01491-x

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• DOI: https://doi.org/10.1038/s41567-021-01491-x

• ### An exceptional mass dance

• Wei Yi

Nature Physics (2022)